Problem: Cannot send MMS text messages (pictures/ videos) when mobile data is turned off.
Solutions (Without Samsung UI, with Samsung UI):
A: Android phones without Samsung UI:
settings > network & internet > Mobile network (or Google Fi) > MMS messages send & receive when mobile data is off > enable. Problem solved.
B: Phones with Samsung UI:
The above setting is NOT available, at least not directly.
Step 1: Download Google Messages. This is just temporary, can delete app again after the fix.
Step 2: Complete Google Messages setup, set as default messaging app.
Step 3: Turn off Mobile data and wifi.
Step 4: Send an MMS message (picture) to yourself or someone else.
Step 5: A pop up will appear saying cannot send message, change settings to allow MMS with data disabled. Click on the popup, then enable the feature in the new settings menu. Problem solved
Note: in step 5, the settings menu you're brought to is called "Mobile networks". However, this is not exactly the same "Mobile networks" that's normally accessible through the phone's settings menu.
It combines some features from Settings> Connections> Mobile networks, & Settings> Connections> Data usage, & also adds the needed toggle mentioned above.